A horizontal position disparity (PD) in a broadband stimulus introduces binocular phase disparities that increase systematically with the spatial frequency (SF) of its spectral components. However, for e.g. with crossed disparities, only phase disparities between 0 and 180 p(hase)deg veridically signal the direction of stimulus depth. Spectral components with phase disparities beyond this veridical range are aliased and signal a reversed direction of depth. Here we investigate how aliased phase disparities are used in the perception of stereoscopic depth.

Observers (N=2) fused a pair of RD images on the 2 sides of a monitor, presented to the 2 eyes using paired polarizers. Each eye's image consisted of a 1-deg inner square (IS) of RDs centered in a 3-deg outer square (OS) of RDs. Identical OSs in the 2 images provided a reference plane for phase disparities that were introduced in the IS. Phase disparities equivalent to −16 to +16 min PD were produced in the 2 ISs by manipulating each image's Fourier phase spectrum. Observers reported if a 4×4 min bright probe was in front or behind the IS in 3 conditions, based on the SF components in the IS: SFs with disparities in the veridical range (1), in the aliased range (2), and all SF components (3). From trial to trial the PD of the probe varied randomly.

The perceived depth of the inner square, obtained from a psychometric function, was identical in the veridical and all-SF conditions for all equivalent disparities. In the aliased condition perceived depth was bi-stable, corresponding to an inner square perceived in front or behind the outer square.

The presence of SF components with aliased phase disparities does not affect the perception of veridical depth in a broadband stimulus. Perception of the veridical direction of depth in one of the bi-stable depth planes when only the aliased SF components are present suggests that the gradient of phase disparity with respect to SF may contribute to the computation of stereoscopic depth.